Production of mg(oh)2



March 5, 1963 G. w. WALDRON ETAL. l3,080,215

PRODUCTION oF Mwm2 Filed Mroh 11. 1960" o/om//fc /fm esfone INVENTORS Geo/ge IV Wo/q/ron Jo/m A. Per/ard HTTORNEYS United States Patentfiice 3,080,215 Patented Mar. 5, 1963 3,080,215 PRODUCTION F Mg(OH)2 George W. Waldron and John Neil Periard, Bay City,

Mich., assignors to The Dow Chemical Company, Mldland, Mich., a corporation of Delaware Filed Mar. 11, 1960, Ser. No. 14,479 11 Claims. (Cl. 23a-201) The invention relates to the production of magnesium hydroxide. It more particularly relates to an improved method of producing magnesium hydroxide by the dolomitic liming of -a magnesium chloride-containing inland brine.

This is a continuation-impart of our copending application S.N. 684,529, filed September 17, 1957, now abandoned.

Magnesium hydroxide is currently produced by a number of processes. One process extensively practiced ernploys limestone as a starting material. The limestone is Calcined to quick lime which is water-slaked to make a calcium hydroxide or slaked lime slurry, the slurry thus made admixed with a magnesium chloride-containing brine in a reaction vessel to form a magnesium hyd-roxide precipitate in suspension, the precipitate subsequently separated from the mother liquor (usually by fil-tration), and the thus recovered precipitate washed with water. It may thereafter be dried if a slurry thereof is not desired. The magnesium hydroxide thus p-roduced is generally satisfactory fo-r uses which require a high purity product, e.g., for the manufacture of periclase type refractory products. Other processes used in the production of m-agnesium hydroxide employ dolomite as a :starting material, which is calcined, the oxides water-slaked, the slaked slurry stored for a convenient period of time and then reacted with a magnesium chloride-containing brine to form magnesium hydroxide in suspension, and the magnesium hydroxide thus made recovered and water-washed.

The use of calcined dolomite in the production of magnesium hydroxide has certain inherent advantages over the use of calcined limestone. Among such advantages are the high yield of magnesium hydroxide when dolomite is employed since it itself, when Calcined, contains about l mole of MgO per mole of CaO' and, therefore, when reacted with a magnesium chloride-containing brine and the magnesium oxide becomes hydrated, yields substantially two times the amount of magnesium hydroxide which would be produced from a reaction of Calcined limestone with an equal quantity of the brine. Dolomite is relatively plentiful, is found in a substantially high degree -of pur-ity, and is often located conveniently near natural brine sources, as for example, in lthe state of Michigan, U.S.A.

Disadvantages, however, have hereto-fore been associated with the production of magnesiumhydnoxide as a precipita-te employing dolomite as a raw material, salient among which have been its poor filterability when separating it as a filter cake from the mother liquor and when dewatering it following washing or following re- `slurrying of the washed cake, the low solids, i.e., the low density of the filter cake produced during filtration (often as low as 30-35 percent solids), and particularly the undesirably high contamination of the product produced from water-slaked dolime unless promptly used, particularly calcium contamination which has also been associated with contamination of the magnesium hydroxide precipitate from ingredients present in the brine, especially chlorides and borates. ing an aqueous slurry of magnesium hydroxide, commonly carried out by employing a rotary vacuum filter Among the larger uses of magnesium hydroxide is the manufacture of periclase-type refractory products which `cannot tolerate an appreciable contamination of the Dewatering refers to concentratmagnesium hydroxide. Diiiiculties arising from contamination by calcium have been particularly troublesome. As much as 1.5 percent calcium oxide in the ignited magnesium hydroxide product is generally considered a maximum contamination for commercial ac ceptance and not more than l percent CaO is preferred. Since a number of large deposits of dolomite are substantially pure, e.g., the Cedarville quarries of Michigan, which by analysis shows it to consist of about 1.03 moles of CaCO3 per mole of MgCO3 with a small percent of inerts, the only concern regarding contamination in the use of such dolomite as a source material is calcium contamination of the magnesium hydroxide.

A desideratum therefore exists for a method of making a magnesium hydroxide product employing dolomite and a MgCl2-containing brine to produce a dense magnesium hydroxide product recoverable by ordinary filtra-tion equipment as a washed filter cake containing at least 45 percent Mg(Ol-l)2 and preferably more than 50 percent Mg(OI-I)2 which is low in impurities.

The invention provides an improved method for the production of magnesium hydroxide which has a high filtrati-on rate for the separation of Mg(OH)2 as a dense filter cake from the mother liquor and a high density cake of high purity when subsequently washed or washed and reslurried and thereafter dewatercd.

The invention accordingly consists essentially of calcining dolomite at not over about l450 C. to form the oxides; slaking the oxides with a s-laking liquid comprising an aqueous calcium chloride solution substantially free of Mg ions and having a specific gravity of at least about 1.07 and preferably not over 1.30 (as measured at about 25 C.) to produce a slaked dolomitic slurry hereinafter referred to as a dolime slurry; intermixing the dolime slurry with a brine containing at least about 2 percent by weight MgCl2 in an amount sufficient to provide an excess of Ca(OH)2 in the dolime slurry over the MgCl2 in the brine for at least about 2 hours for brines of low MgCl2 concentration and at least about 3 hours for 1higher concentration brine at a temperature of between 40 and 80 C. to form a precipitate of magnesium hydroxide in suspension which is readily tilterable; separating the thus suspended magnesium hydroxide from the mother liquor, as by filtration to produce a high density filter cake; and water-washing the filter cake thus made to produce magnesium hydroxide,- sometimes called magnesium hydrate, of high bulk density and high purity wherein the calcium, boron, and chlorine contamination is sufficiently low for the general use thereof, especially for use in the production of periclase by ignition.

The attached drawing illustrates schematically a liow sheet which represents one mode of practicing the invention.

The drawing showing a pulverizer which is used when higher specific gravity calcium chloride brine is employed, and a recycle line from t-he precipitator to the slaker, which is used when the precipitator slurry is employed as the slaking liquid, is clearly illustrative of a mode of practicing the invention and not to be construed as a limitation.

Referring to the drawing in greater detail there is schematically shown conveyor 10 for conveying dolomite into kiln or calciner 12 from a source of dolomite, not shown. Calcined dolomite from kiln 12 is conveyed by conveyor 14 to cooler 16. From cooler 16, conveyor 18 carries the Calcined dolomite (dolime) to a crusher or pulverizer 20 from whence the crushed dolime is taken by conveyor 22 into slaker 24 provided with stirrer 25. Overflow line 26 leads from slaker 24 through pump P,

into the top of a reaction vessel, hereinafter called a line 30, from a source of brine, not shown, also empties into the top of precipitator 28. Precipitator slurry line 31 leads from an outlet near the top of precipitator 28 and bifurcates into two lines: recycleline 32 leading back to slaker 24 and line 33 leading to a filter pit 34, e.g., one comprising a rack of Moore filters. Filtrate from filter 34 exits through line 35 to evaporator 36. Overhead portable lift 38 moves the filters, to which magnesium hydroxide cake is adhering, from filter pit 34 to wash pit 40 into which fresh Water from a source not shown is admitted. Line 41 carries away wash filtrate. Overhead portable lift 42 moves the Moore filters, to which washed magnesium hydroxide cake is still adhering, to storage pit 44 where the magnesium hydroxide is removed and deposited. By suitable arrangement, the same hoist may serve as both 38 and 42.

In pra-cticing the invention, dolomite is calcined to drive off CO2 to yield oxides of calcium and magnesium, i.e., dolime. Calcination is usually carried on at between 1150 and 1450 C. The dolime is thereafter crushed to a suitable size. The particle size is not highly important but is usually less than 21A-inches. It may include pieces which pass through a 200 mesh sieve or even smaller. The particle size recommended depends in part upon the specific gravity of the calcium chloride slaking liquid. For example, when employing a slaking liquid having a specific gravity greater than 1.15 but not over about 1.30, it is preferred that the dolime be pulverized so that at least'about 90 percent will pass through a 420 mesh screen and preferably that it contains particles of such *size that at least 25 percent will pass through a 1.00 meshscreen and at least about l percent through a `200 mesh screen. When employing a specific gravity of between about 1.07 and 1.15, no particular advantage is seen in employing the finely pulverized dolime but larger pieces are satisfactory, pieces ranging between 1/2 inch and 2% inches usually being used. It has been observed that when the slaking liquid has a specific gravity greater than about 1.15 the dolime slurry produced in the slaker tends to contain some undispersed lumps, due apparently to the slow slaking rate in the higher specific gravity calcium chloride solutions. A calcium chloride slaking liquid having a specific gravity greater than about 1.3 may be used but slaking liquids having specic gravities greater than 1.3 indicate a need for a smaller particle size dolime and a longer slaking time with no apparent advantage since available brines employed produce a mother liquor for slaking having a specic gravity of. not over 1.3.

The slaking time recommended is between 4 minutes and 3 hours dependent in part on the temperature and 'type of agitation provided in the slaker. The preferred time is between 0.1 and 0.5 hour since the shorter time produces a higher density cake having better filtering properties. For example, a slaking period of 0.1 hour produced a 63 percent solids cake whereas a 1.5 hour slaking period produced a 55 percent solids cake. However, a satisfactory density cake can be produced emfploying a slaking time of 4 to 6 hours or even longer. Itis clear, however, that in view of no apparent advantage in 'a prolonged slaking period, efiiciency and practicality would indicate the employment of the shorter kslaking period. The temperature in the slaker is usually between 40 and 90 C. The slaker may be any one of known types of mixers, e.g., either a rotating cylindrical type slaker provided with bafes or balls or a tank type (as shown in the drawing) provided with a high speed agitator, e.g., of the impeller type. The slaking liquid is an aqueous solution containing suhicient calcium chloride to provide the specific gravity set out above. It may be a freshly made up solution, or calcium chloride mother liquor, either the slurry containing magnesium hydroxide suspended therein from the precipitator or the substantially clear filtrate recycled back from the filtration stepin the operation. lf the recycled calcium chloride filtrate or mother liquor of the magnesium hydroxide slurry has a higher specific gravity than is desired, it may be readily diluted to the desired specific gravity by merely introducing a iiow of water into the slaker along with the calcium chloride filtrate or slurry.

The ratio of slaking liquid to dolime in the slaker should be such as to provide adequate intermixing of the dolime and calcium chloride solution but avoid recycling an unnecessarily large volume of liquid, and to result in a dolime slurry which is easily pumped. The slaker feed ratio is usually between about 0.8 and 3.3 pounds of dolime per gallon of slaking liquid. Dolime concentrations below this ratio are acceptable but represent an undesirably large recycle stream. Higher concentrations than this ratio in the slaker tend to cause the temperature thereof to rise above the C. upper limit. At such higher temperatures, there appears to be a tendency to form calcium oxychloride crystals which coat the slaker surface.

The dolime slurry is led from the slaker and introduced into the precipitator concurrently with a magnesium chloride-containing brine. The dolime slurry is preferably led directly to the precipitator without undue delay after it is prepared. Where the slurry is not transferred to the precipitator for a period of several hours there is a tendency for the final magnesium hydroxide product to have a lower density than is desirable. The precipitator, eg., 28, is usually an open top vessel equipped with baffles and an agitator, e.g., 29, which is of such size and rotates at a speed suiciently high to keep the contents' of the precipitator well mixed. To illustrate suitable agitation, a 3D0-gallon precipitator (having a diameter of 48 inches and a depth of about 38 inches) may employ an agitator which requires a power load of between 0.5 and 0.6 net Watt per gallon of precipitation slurry in the precipitator. In larger tanks, such as one having a diameter of 40 feet and a depth of 40 feet, the power required for agitating is between about 0.25 and 0.3 net watt per gallon. By net watt is meant the power employed for agitation exclusive of that necessary to turn the stirring mechanism at the same speed in an empty precipitator. For best results, both the magnesium chloride-containing brine and thedolime slurry are fed through separate pipes which discharge into the precipitator, preferably at positions on opposite sides of the agitator so that both the dolime and the brine are effectively diluted with precipitair slurry before intermixing and reacting with each ot er. An excess of calcium hydroxide from the dolime slurry, i.e.,' more than 1 mole of Ca(OH)2 per mole of MgCl2, 1s always maintained in the precipitator. A very slight excess is preferred. Methods of maintaining the desired excess of Ca(OH)2 consists usually of periodically checking either the vsoluble alkalinity or the pH of the contents of the precipitator.

A recommended practice-is to determine quantitatively the relative amounts of brine and dolime necessary to lnsure a slight excess of Ca(OH)2 over the MgCl2 and thereafter correlate the desired excess with either the pH value or the `soluble alkalinity value of the mother liquor. Soluble alkalinity is readily ascertained by titratlng a given volume of mother liquor with 0.1 N hydrochloric acid to a phenolphthalein end point and recording the soluble alkalinity as the number of cc. of 0.1 N hydrochloric acid required. The residence or dwell time, herein usually denoted as inventory time, in the precipitator is an important factor in producing magnesium hydroxide 0f high quality according to the in- Ventron. InventoryV time in the precipitator may be expressed as the average time in hoursthat a pound of Mg(OH)2 is suspended in the precipitator. In other words, it is equal to the pounds of Mg(OH)2 in suspension in the precipitator divided by the pounds of Mg( OvH)2 being produced per hour. The inventory time may also be based on volume and expressed as the average time in hours obtained by dividing the volume of the precipitator in gallons by the gallons of overflow slurry produced per hour in the practice of the invention. Between 3 and 20 hours are usually employed. Where the inventory time is less than 2 hours, the rate of precipitation in proportion to the seed crystals of magnesium hydroxide in the precipitator appears to be too high to form the desired particles for filtration and dewatering. As a result thereof, the cake density decreases sharply as the time is shortened below about 2 hours. Too short an inventory time, furthermore, does not provide suicient time for the larger dolime particles to react with the magnesium chloride in the brine. Therefore, an increase in calcium contamination in the magnesium hydroxide product results. An inventory time beyond 25 hours is not recommended because it entails larger precipitators and higher agitation power costs without commensurable returns.

The temperature maintained in the precipitator is between 40 and about 80 C. A small amount of steam may advantageously be introduced into the precipitator to maintain a substantially constant temperature for more easily controlling the precipitation step. At temperatures of less than 40 C. the reaction time is unsatisfactorily slow. Temperatures above 80 C. may be employed but heating costs become significant at such temperatures a-nd as the temperatures approach the boiling point, the increased evaporation from an open-top precipitator is undesirable.

A convincing qualitative test for insurance that there is an excess Ca'(OH)2 in the precipitator over the MgCl2 necessary to react therewith for the purposes of confirming the pH or soluble alkalinity tests, consists of testing a small portion of the mother liquor from the slurry by adding thereto a few drops of the MgCl2-containing brine. The presence of a small amount of soluble Ca(OH)2 slowly produces a milky or slightly turbid condition within about a minute. An amount of Ca(O1-I)2 is thereby shown to be present in the precipitator in excess of the stoichiometric equivalent of the MgCl2 which, stated in other words, requires that substantially all the MgCl2 in the precipitator be completely reacted to form magnesium hydroxide leaving a slight excess of unreacted Ca(OH)2 in solution. A large excess of Ca(OH)2 is not employed because, with an increase thereof beyond that necessary to react completely with the MgCl2 present to form a precipitate of magnesium hydroxide, there is an increase in the calcium impurities in the product being produced.

The magnesium hydroxide slurry so produced in the precipitator is usually led off through an over-flow line positioned near the top of the precipitator as shown in item Z8 of the drawing, Such overflow line in commercial practice leads into a filtration means, e.g., a vacuum type filter, and there filtered leaving the magnesium hydroxide as a cake. The filtrate is drawn off and the salts contained therein, particularly the CaCl2, are recovered therefrom. The Mg(OH)2 is then water-Washed, and often reslurried for convenience in transportatiom and subsequently dewatered prior to use. For purposes of illustration, the slurry is recovered and the filtrate is usually merely discarded. The cake thus recovered, as in commercial practice, is washed with water to remove entrained mother liquor and thereafter reslurried to make it pumpable and subsequently dewatered to a satisfactory concentration, e.g., a cake having a total solids of at least 45 percent and preferably 55 to 60 percent.

In the practice of the invention, the slaking liquid may be any CaClz-containing aqueous liquid, e.g., a freshly made up CaCl2 aqueous solution. A preferred practice of the invention, however, employs either some of the slurry from the precipitator which is recycled back into the slaker, carrying along the magnesium hydroxide suspended therein in the recycling operation or employs 6 the portion of the filtrate from the filtration operation which is piped back to the slaker in a similar manner to that of the slurry.

The following series of runs were made, including runs which are illustrative of the practice of the invention, designated examples, and some which were not made according to the invention but for purposes of comparison and are so designated.

A typical analysis of the dolomite employed in all runs, when ignited to the oxides, showed the following percentage composition: 57.5 percent CaO, 40.0 percent MgO, and 2.5 percent inerts An analysis of a typical brine employed in the runs of Series One showed the following salts dissolved therein by weight: 16.7 percent CaCl2, 9.4 percent MgCl2, 2.70 percent NaCl, 0.96 percent KCl, and lesser amounts of other salts including those of Sr, Li, B, and Fe. Its specifc gravity was 1.28.

SERIES ONE This series was made to illustrate the effects of employing a CaClz-containing slaking liquid according to the invention in contrast to employing water as a slaking liquid in the production of magnesium hydroxide from dolomite and a MgCl2-containing brine. Run A employed water for purposes of comparison. Examplesl and v2 employed a CaCl2-containing slaking liquid and differed from one another in that the liquid employed in Example l had a specific gravity of over 1.07 but less than 1.15 and that employed in Example 2 had a specific gravity of between 1.15 aad 1.3.

Calcined dolime and a slaking liquid, either the water or the aqueous CaCl2 liquid, were fed into a continuously operated rotating cylindrical slaker provided with baffles in run A and Example l and in all other runs and examples into a slaker of the type illustrated schematically in the drawing by numeral 24. Feed rates were maintained to provide the ratios of dolime to liquid and the slaking inventory times are shown in Table I set out hereinafter. The slaked dolime slurry was led from the slaker to a precipitator, one type shown schematically in the drawing a-s numeral 28. Concurrently with the dolime slurry, brine was admitted at a controlled rate through line 30 to the precipitator, the brine rate being controlled to maintain an excess of Ca(OH)2 over the stoichiometric quantity required to react with the MgCl2 in the brine. Precipitated magnesium hydroxide in suspension was formed in the precipitator leaving a substantially magnesium-free mother liquor. Overflow slurry from precipitator 28 owed out through line 31, a portion of which returned to slaker 24 through line 32 to provide slaking liquid in Examples l and 2, and the balance of the slurrydrawn off through line 33 for subsequent filtration for the recovery of magnesium hydroxide cake from the CaClz-containing mother liquor. In Example 1 some water was fed into the slaker along with the recycled slurry to reduce the specific gravity of the slaking liquid. (The specific gravity of the slaking liquid refers to thatof the mother liquor only aud does not consider the suspended magnesium hydroxide therein which does not enter into the slaking action but is merely carried along for convenience.) The magnesium hydroxide precipitate was recovered a-s a cake as by periodically passing the slurry into a Buchner funnel subjected to a reduced 4absolute pressure of 6 inches of Hg instead of the commercial filters suggested by item 34 of the drawing. The magnesium hydroxide cake is water-Washed on the funnel to provide a cake free of mother liquor. Since, in common practice, the washed cake is usually admixed with 'enough water to produce a pumpable slurry, the washed ycake in this series was therefore sometimes slurried and subsequently dewatered, employing the dewatering test referred to in footnote (8) of Table I. The filtrate was discarded, although in practical operations the filtrate, is usually subjected to evaporation, e.g., in evaporator 36,

associe.

to recover the dissolved salts therein, particularly the CaClg.

The slaking and precipitating conditions and the more important characteristics of the precipitated product prod sientas Two A series of runs was made to illustrate the effect of variations in inventory time in the precipitator. The series consists of two comparative runs, designated B and duced are shown in Table I set out following Series Two C and Examples 3, 4, and 5 of the invention. Dolime below. employed in the runs of Series Two showed the same Ari @XamrlaOn 0f the leSUitS 0f IUD A anti EXemPles analysis as that set out above and employed in Series One. 1 and 2 of Table I shows a number of facts which are The brine, however7 employed in Example 3 of Series pertinent t0 the superiority 0f tile method 0i the iUVel- Two was the same as that employed in Series One, for tiOIl employing Cacia Solution as tile Siaitiiig iitlliid instead which a typical analysis is set out above. The brine ernof water, among which are: improved filtration rate and ployod in the remaining runs of Series TWO, viz comhigher Percent Mg(OH)2 ill the Cake Produced during parative runs B and C aud Examples 4 and 5, showed by filtration and imPTOVed CieWatei'iIlg fate and higher Pefa typical analysis to contain the following salts dissolved cent solids after admixing the washed cake with water therein, by weight: 17.5 percent CaCl2, 3.3 percent (which was done to simulate frequent actual practice). MgCl2, 5,0 percent NaCl, 1.8 percent of KCl, and lesser The percent calcium contamination in the final cake is amounts of other salts including those of Sr, Li, B, and comparable for the three runs, eg., (162-094 CaO when Fe. Its specific gravity was 1.26. The procedure foiemploying Water as COmpared t0 0.5419 C210 When mlowed in Series Two for both blank runs and examples ploying a CaClz-containing slaking liquid having specitic was similar to Examples l and 2 of Series One except that gravity of 1.3. It should be noted that blank A, which 2O in blanks B and C, less than the inventory time consid- WBS run fOr COITlpafatVe PUIPOSeS (emPiOS/Hlg Water 21S ered satisfactory for the purposes of the invention was the slaking liquid rather tha-n a CaClZ solution as required followed whereas in Examples 3, 4, Vand 5, the inventory bv the Practice 0f the invention), although not a practice time was sufficient to insure the attainment of the objecof the invention, was not in accordance with general practives of the invention. lt will be observed in evaluating tice, and gave a Mg(OH)2 product of low calcium im- 25 the runs of this series that an inventory time in the prepurity. Blank A wa-s not fully satisfactory because of cipitator of about 2 hours is satisfactory when the brine its less desirable lterability. Blank A differed from genemployed contains about 9.0-9.5 percent MgCl2 whereas eral practice in that the dolime slurry was passed directly at least about 3 hours precipitator inventory time is reand without delay into the precipitator to avoid any subquired when the MgCl2 content of the brine is about stantial slaking of the MgO of the dolime prior to entering 3.0-3.5 percent. the precipitator as described in our copending application The pertinent operating conditions and results are set S.N. 684,421, tiled September 17, 1957. out in Table I. i

Table I Series one Series two Examples Comparative Examples Compararuns tive run A Duration of rim, days 18 7 2 7 Temperature in slakcr, C.- 75-78 70-75 74-77 70 71-73 74-91 65-70 63-77 Slaker inventory time, minutes 20 8-9 13 6.- 7-8 4-9 Pound of dolime per gallon slaking 2. 4-28 1.2-1.3 1.4-1.7 1.1-1.2 1.1-1.2 1. 5-1.9 1. ii-1.7 l 3-1.9

l U1 Paiiicle size of dolime to slaker 2....- (e) (b) (b) (i) (b) (b) (b) Kind of slaker liquid feed 3 (e) (d) (s) (e) (e) (2) (d) sp. gr. ofsiakeriiquid feed at 25 C-.. 1. o L09-1.1i 1. 3 1.27 1. 27 .3 1. 27 i iti-1.14 Temperature in prccipitator, C 57 57 67 57 57 77 51 57 Precipitator inventory time, hours.. 15 14-15 20-21 1.3 2.1 2.1 6. 5 3. 4 Rztito oI4Ca(OH)z to MgCl; in precipi- (l) (i) (i) (i) (i) (i) (i) (i) a Ol. Pounds Mgwnppmduct per hour..- 15-18 16-17 io-i 7s 5.1 iti-17 15. 516.5 eso-69o sp. gr. oifiitratefroni filter et2s C-.. mos-1.216 1203-1211 1 zii-1.32 1.27 1.27 L27-1.29 1. 24-1. 26 1.24 Filtrate rate oi slurry, ga1./hr./sq. ft- 7-9 10-11 14-19 3() 3 10-18 14-16 15-17 Percent Mg OEllz in washed cake 1.... 51. 5-52. 8 59-60 59-61 35 3G 58-64 56-58 563-59 Percent CaO in ignited washed cake... 0. 62-0. 94 0. 8-0. 9 0. 5-0. 9 1. 4 1. 2 0` 7-1. 6 0. 7-1` 0 1. 0-107 Percent Mg(OH)s iii water slurrled 46 52 51 (ir) 1.5.5 51.6

washed cake. 7 Dewatering rate in pounds of 25-50 211 250-325 (s) (g) (s) 142 144 Mg(OH)2/hr./it.i. Percent Mg (OHM in dewatcrcd cake 8 56 61. 5 60. 6 (E) (E) (S) 58. 2 63. 3

l Calculated as volumetric inventory as follows: gallons in slaker X60.

2 Orushed, containing up to it" percent through a 20 mesh sieve, at least 67 3 Slurry means precipitator slurry, primarily CaCl 4 Soluble alkalinity as cc. ni N /10 HC1 required to equivalent amount of each employing this brine gallons/hour through slaker size particles with 23-34 percent remaining on a No. 20 mesh sieve.

can be calculated to show that an additional 0.033 percent MgCl2 would have been necessary to react with all the Ca(OH)z present.

Excess Ca(Ol[)2 in Series Two was shown by solutlon to the phenolphthalein end 7 In accordance with general quent-ly dewatered. The final solids of adding a few which produced milky turbidity and thereafter maintaining precipitator slurry through a Buchner tunnel with 24 6 Percent Mg(OH)n determined by titration of a weighed drops of MgCl2 brine to samples of clear prccipitator mother liquor a solubility alkalinity which insured this slight Ca(OH)2 excess.

sample of cake with N/l HC1 and back tltrating with N/l NaOH point and calculating the percent Mg(O1-I)z as follows:

0.02917Xriet cc. of 1.0 N HC1 grams of sample X practice. Washed lter cake had Water added to make a thick but pumpable slurry and was subsethe dewatercd cake obtainable without a protracted dewatei'ing o1 eration is usually just slightly higher than that obtained in thc washed filter cake.

5 Determined according by McGraw-Hill, New York. New York.

Crushed. b Pulveiized. C Water. MgCl2 was maintained. l Not determined.

to standard dewateririg test as described in Perrys d Water and slurry.

Handbook, 3rd Edition, 1950, page 969, published Slurry.

"Pulverized, at leastv percent through a 100 mesh sieve. and at least 42 percent through a 200 mesh sieve.

2 solution containing Mg(OH)r in suspension.

titrate 100 cc. oi mother liquor to a plienolphthalein end point was 10-12 in Series 011e. This conclusively shows au excess oi @MOHM from the dolime over the MgCl2 from the brine because a stoichiomctricA gives a solubili ty alkalinity ol' 2 cc. Therefore, the excess of about 9 cc. alkalinityv mercury vacuum, to a 1" thick recovered.

f A slight excess of Ca(OH)z over An evaluation ofthe table clearly shows that a dolime slurry prepared by slaking dolime with a calcium chloridecontaining solution or slurry and the slaked dolime reacted with a MgCl2-containing brine maintaining an excess of calcium hydro-xide over the MgCl2 in the reaction vessel and an inventory time of at least 2 hours for the brine containing at least about 9 percent MgCl2 and an inventory time of at least about 3 hours for the brine containing not over about 3.5 percent MgCl2 produces a magnesium hydroxide product which is more readily filtered, washed and dewatered, and is less calcium contaminated. Furthermore, it can be easily seen that the byproduct CaCl2-containing filtrate, is suitable for direct recovery of CaClz therefrom whereas the presence of MgCl2 in the filtrate is objectionable since it decomposes during drying to form HCl and MgO. The MgO thus formed therein produces a turbid CaClz solution when the CaCl2 productv is subsequently dissolved in water. To render the filtrate, which contains unreacted MgCl2, suitable for general use, it is necessary to add an alkali and reiilter, a costly and denitely undesirable step. Furthermore, magnesium hydroxide formed by a reaction wherein the Ca(OH)2 is not in excess of the MgCl2, a product is formed from which it is dicult it not impossible to remove chlorides by ordinary washing thereby rendering the product unfit at least for refractory uses.

It is also clear that where boron compounds appear in the brine, as is not infrequent, a failure to have a slight excess of Ca( OH )2 in the precipitator in some way causes an increase in adsorption of both boron compounds and chlorine compounds in the magnesium hydroxide product. Tests have been run to show that when there is no excess of Ca(OH)2 over the MgCl2 in the precipitator, the boron and chlorine content of the Mg(OI-I)2 product produced was definitely increased over such contamination when an excess of Ca(OH)2 was maintained. For example, where it was known that the stoichiometric quantity of Ca(OH)2 for a'specific brine (as determined by measuring the solu ble alkalinity of 100 cc. of precipitator mother liquor by titrating it with 0.1 N hydrochloric acid to a phenolphthalein end point) gave a soluble alkalinity of cc. of the acid, such 20 cc. soluble alkalinity value resulted in a boron content in excess of 0.9 percent, calculated as percent B203, based on the Weight of the ignited Mg(OH)2 product made. But when the soluble alkalinity of 100 cc. of the precipitator mother liquor required 24 cc. of 0.1 N acid, the boron contamination of the ignited Mg(OH)2 was reduced to less than 0.5 percent and when the soluble alkalinity of 100 cc. of the precipitator mother liquor was increased to require cc. of 0.1 N acid (indicating slightly greater excess of Ca(OH)2 over MgCl2) the boron content calculated as percent B203 was less than 0.25 percent.

Similarly, when the soluble alkalinity of 100 cc. of the precipitator mother liquor (employing the same brine as above and titi-ated as above) required 20 cc. of 0.1 N hydrochloric acid, the tot-al chlorine contamination cal culated a-s percent CaCl2, of the dried Mg(OH)2 product, exceeded 0.62 percent. When the soluble alkalinity of 100 cc. of precipitator mother liquor was increased to require 24 cc. of 0.1 N hydrochloric acid, the chlorine contamination, as CaCl2, was less than 0.39 percent of the dried Mg(OH)2 product.

Therefore, among the 4advantages of the practice of the invention is the decreased contamination by boron and chlorine compounds both of which are definitely undesirable for use of magnesium hydroxide `in products requiring high purity ingredients.

.The following example was run to illustrate further the practice of the invention and to show in detail the significance of practicing the invention as described.

EXAMPLE 6 Dolomite having the composition set forth hereinbefore l0 and brine having the composition set forth hereinbefore were employed in this example.

The example was run for 20 days during which periodic checks and tests were run to ascertain operating conditions and evaluate the quality of the magnesium hydroxide cake being produced.

The dolomite was fed into a rotary cylindrical kiln represented by item 12 of the drawing which was inclined downwardly toward the outlet end and tired by a mixture of pulverized coal and natural g-as forced in the outlet end thereof. The kiln was operated between 1300 and 1365 C. Dolirne thus produced was carried by conveyor 14 to cooler 16 where it was cooled to below about C. The cooled dolime was conveyed by a screw conveyor to pulverizer 20 which subdivided the dolime into a powder which was periodically sampled and tested for screen analysis. The range of percent and the average percent of various size ranges for a number of samples of pulverized dol-inte used are set out bel-ow and may be considered typical for use with high specific gravity brine.

Weight percent Mesh (U.S. Standard Sieve Series) Range Average It can be seen from the above average percents that smaller than a mesh sieve is l8|68 or 86 percent.

The pulverized dolime was conveyed to slaker 24 where the dolime was slaked with the CaClZ-containing slurry entering from lime 32. The mixing action was aided by a high-speed stirrer. The principal slaker conditions employed in this example consisted of a flow of dolime of between about 14 and 16 pounds per hour and a flow of precipitator slurry thereto of about 9.75 gallons per hour, a slaker temperature of about 75 C. The slaker yinventory time was 1.1 hours (calculated by adding the dolime and slaker liquid, i.e., by dividing the operating capacity of the slaker by the input per hour). The principal precipitator conditions consisted of admitting an loverow from 'the slaker into a precipitator having a capacity of 300` gallons and simultaneously therewith a- The normality or `the magnesium hydroxide slurry pro` duced as about 4.5 N.

Overflow line 31 from precipitator 28, leading into line 32. carried a portion of precipitator slurry backto slaker 24 to provide the CaCl2 slaking liquid.

The mathematical evaluation of the precipitator conditions of Example 6 set out below shows that (A) an excess of Ca(OH)2 over the MgCl2 therein was maintained and (B) an inventory time in excess of the minimum required by the practice of the invention was maintained (the MgO content of the calcined dolomitic aqueous slurry, does not enter into the reaction and, accordingly, is not a factor in the calculation of the reaction quantities of dolime and MgCl2-containing brine in the precipitator).

(A) The ratio of Ca(OH)2, as CaO, may be calculated as follows: l

(1) Since the rates of ow during the operations werev sondare 15.0 pounds/hour of dolime and 14.1 gallons/hour of brine; (2) since [the specic gravity of the brine was 1.28 and the weight of a gallon of water is 8.345 pounds/ gallon, the weight of a gallon of brine is readily determined by multiplying 1.28 8.345; (3) since the fractional weight of MgCl2 in the brine is 0.094 MgCl-2 or 9.40 percent; (4) since the analysis of the dolime was 57.5 weight percent CaO and 40.0 weight percent MgO; (5) since the molecular weight 'of MgCl2 is and the molecular weight of Ca() is 8; then by employing the italicized values above as the numerator, the underlined values as the denominator, canceling out, and calculating thus:

14.1 'ga-l- 8345 lbf moles Fr-X128 X-E-Xogtl: NIgClgXm it is shown that an average oir 0.1478 mole MgCl2/hour was fed into `the preeipitator. Further calculating thus:

15.0 lle: moles 0575 CaO 56 08 b1 it is further shown that an average of 0.1538 mole CaO/ hour was fed into the precipitator. subtracting as follows:

0.15%8 molle (IsaaCl 006 -0.14 8 mo e g' 2 0.0060 mole excess of Cao or 0.1478X 100 ==4.05% excess CaO 300 golf 'f5-gm Ol' 20 hOUIS Proceeding with Example 6 following the reaction in the precipitator, the major portion of the overow slurry from the precipitator was carried by line 33 to a collecting vessel and periodically passed into a Buchner funnel where the suspended magnesium hydroxide was removed for evaluation. The slurry filtered rapidly, having a filter rate of 14 to 18 gal. per hr. per ft?, to produce a high density washed cake containing 53 to 58 percent Mg(OH)2 solids. (In commercial operations, the magnesium hydroxide slurry would have followed the general course suggested by the Moore tilters, washer, and evaporator shown in the drawing.) The magnesium hydroxide recovered on the Buchner was washed to remove entrained CaClZ mother liquor and thereafter dried, ignited to dryness, and analyzed for oxides of Ca, Si, Al, and Fe. Twelve samples in all were taken on different days. The highest Ca content, calculated as Ca() of any sample was 0.853 percent, the lowest 0.522 percent. A total of ve samples taken on different days showed the sum of the oxides of Al and Fe to vary between 0.95 and 0.53 and SiOZ to vary between 0.87 and 0.67. The analysis of the magnesium hydroxide produced, therefore, showed it to be of high purity suitable for meeting premium quality specifications for use in making periclase-type products.

Thus it is seen that by calcining dolomite, preferably at a temperature between 1150 and 1450 C., slaking the calcined dolomite with a liquid comprising an aqueous solution of CaCl2 having a specific gravity of at least 1-.07 and preferably between 1.15 and 1.3 (providing the dolomite is of a satisfactory neness as aforedescribed), at atemperature of between 30 and 90 C. and usually between 0.1 and 1.5 hours to produce a slaked dolime slurry, intermixing the slaked dolime slurry with a MgCl2-containing brine containing at least 2 percent by weight MgCl2 at a temperature between 40 and 80 C. for at least 3 hours when the brine employed contains not over about 3.5 percent MgCl2 and at least 2 hours when the brine contains higher percents of MgCl2 while maintaining an excess of Ca(OH)2 over the MgCl2, which is preferably a slight excess, a magnesium hydroxide slurry is produced from which the precipitate is readily separated by filtration, readily washed, and readily dewatered (if reslurried with water following washing) to yield to a high density magnesium hydroxide product of high purity suitable for uses requiring less than 1.5 and even less than 1.0 percent Ca therein calculated as percent CaO in the dehydrated ignited cake, and a valuable substantially magnesiumfree CaClz filtrate.

Having described the invention, what is claimed and desired to be protected by Letters Patent is:

1. The process of producing magnesium hydroxide calce of high density and high purity which may be slurried in water and readily dewatered by filtration means consisting essentially of calcining dolomite to produce dolime, slaking the dolime with a slaking liquid comprising an aqueous CaCl?A solution substantially free of Mg ions and the liquid portion thereof having a specific gravity of at least 1.07, at a temperature of between 40 and C. forl at least about 4 minutes, to produce a slaked dolime slurry, and intermixing the dolime slurry so produced with a brine containing at least about 2 percent by. weight ofMgClz for a mixing periodvof at least about 2 hours at a temperature of between 40 and about 80 C., the amount of the dolime slurry being suicient to provide more than one mole of Ca(OI-l)2 per mole of MgCl2 to produce magnesium hydroxide suspended in a CaClz-containing brine, and recovering the thus produced magnesium hydroxide.

2. The method of claim 1 wherein the dolime has a particle size such that at least about 90 percent will pass through a No. 20 mesh sieve, at least about 25 percent will pass through a mesh sieve, and at least about 15 percent will pass through a 200 mesh sieve. i'

3. The method of claim 1 wherein the dolomite has a particle size of not over about 21/2 inches and the aqueous CaCl2 solution has a specific gravity of at least 1.07 and not over about 1.15.

4. The method of claim 1 wherein the calcining temperature is between 1l40 and 1450" and the slaking period is between 0.1 and 0.5 hour.

5. The method of claim 1 wherein the slaked dolime slurry is brought into contact with the brine for intermixture and reaction therewith within about 90 minutes after the slaking period.

6. The method of claim 5 wherein said reaction is carried on at a temperature between about 60 and 75 C.

7. In a method of producing magnesium hydroxide from calcined dolomite and an aqueous magnesium chloride solution the improvement consisting of slaking the calcined dolomite with a liquid containing CaCl2 in solution and substantially free of Mg ions, intermixing the resulting mixture of slaked calcined dolomite and aqueous solution with a brine containing at least about 2 percent by weight of MgCl2 for at least about 2 hours, when the brine employed contains more than 3.5 percent by weight MgCl2 and an inventory time of at least 2 hours when the brine contains not more than 3.5 percent by weight MgCl2, while maintaining a slight stoichiometric excess of Ca(OH)2 over the MgCl2 therein, separating the magnesium hydroxide so produced from the resulting CaClacontaining mother liquor, water washing the separated magnesium hydroxide, and removing from the separated magnesium hydroxide excess water to produce a high density magnesium hydroxide which is low in Ca, Cl, and B contamination.

.8. The method of claim 7 wherein the slight excess of 13 Ca(OI-I)2 is determined by conducting periodic soluble alkalinity tests on the mother liquor 9. The method of producing magnesium hydroxide from particulate dolime and a MgCl2-containing brine consisting essentially of slaking the dolime with mother liquor produced when the slaked dolime is subsequently reacted With the MgCl2-containing brine containing at least 2 percent by Weight MgCl-2, said mother liquor having a specific gravity between about 1.07 and 1.15 when the dolime has a particle size of less than 21A inches and having a specific gravity of between 1.07 and about 1.30 when the dolime has `a particle size of such that at least 90 percent will pass through a No. 2()i mesh sieve, 25 percent through a 10()l mesh sieve, and 15 percent through a 200 mesh sieve, intermixing the slaked dolime with the MgCl2 brine for at least 2 hours when the brine has a MgCl2 content of over 3.5 percent by Weight and at least about 3 hours when the brine has a MgCl2 content of less than 3.5 percent, at a temperature between 40 and 90 C., While maintaining an excess of Ca(OH)2 over the stoichiometric quantity of MgCl2 required to react with the Ca(-OH)2, to produce a slurry of magnesium hydroxide suspended in the CaClg-containing mother liquor, the magnesium hydroxide separated from the mother liquor, and Water-Washing the magnesium hydroxide separated.

10. The method of claim 9 wherein the slaking liquid is the mother liquor containing magnesium hydroxide suspended therein.

11. The method of claim 9 wherein the slaking liquid is the mother liquor recovered as ltrate during the separation of magnesium hydroxide therefrom.

References Cited in the le of this patent UNITED STATES PATENTS 2,465,264 Pike Mal'. 22, 1949 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No., 3,080,215 March 5i 1963 George W. Waldron et al*7 d" that error appears in the above numbered pat- It s hereby certifie nd that the said Letters Patent shouldtread as ent requiring correction a corrected below.

Column l2, line 65, for "2 hours" read 3 hours Signed and sealed this 19th day'of November 1963.,

` (SEAL) Attest:

EL WIN L REYNOLDS ERNEST W. SWIDER Attesting Officer AC lling Commissioner of Patents 

1. THE PROCESS OF PRODUCING MAGNESIUM HYDROXIDE CAKE OF HIGH DENSITY AND HIGH PURITY WHICH MAY BE SLURRIED IN WATER AND READILY DEWATERED BY FILTRATION MEANS CONSISTING ESSENTIALLY OF CLACINING DOLOMITE TO PRODUCE DOLIME, SLAKING THE DOLIME WITH A SLAKING LIQUID LIQUID COMPRISING AN AQUEOUS CAC12 SOLUTION SUBSTANTIALLY FREE OF MG IONS AND THE LIQUID PORTION THEREOF HAVING A SPECIFIC GRAVITY OF AT LEAST 1.07, AT A TEMPERATURE OF BETWEEN 40* AND 90* C. FOR AT LEAST ABOUT 4 MINUTES, TO PRODUCE A SLAKED DOLINE SLURRY, AND INTERMIXING THE DOLIME SLURRY SO PRODUCED WITH A BRINE CONTAINING AT LEAST ABOUT 2 PERCENT BY WEIGHT OF MGC12 FOR A MIXING PERIOD OF AT LEAST ABOUT E HOURS AT A TEMPERATURE OF BETWEEN 40* AND ABOUT 80* C., THE AMOUNT OF THE DOLIME SLURRY BEING SUFFICIENT TO PROVIDE MORE THAN ONE MOLE OF CA(OH)2 PER MOLE OF MGL2 TO PRODUCE MAGNESIUM HYDROXIDE SUSPENDED IN A CALC2-CONTAINING BRINE, AND RECOVERING THE THUS PRODUCED MAGNESIUM HYDROXIDE. 